This invention relates to a process for treating waste slags produced in the desulfurisation and/or dephosphorisation of molten pig iron with alkali carbonates thereby to recover the alkali.
Generally, pig iron includes 3.7-4.8% by weight of C, 0.3-1.0% by weight of Si, 0.2-0.8% by weight of Mn, 0.02-0.05% by weight of S and 0.10-0.18% by weight of P. The presence of sulphur results in the lowering of hot working performance and impact strength of steel and the presence of phosphorous brings about deterioration of a property such as toughness on steel. Accordingly, it is desirable that the sulphur and phosphorus in pig iron are removed as much as possible for a high grade steel.
The desulfurisation of molten pig iron has hitherto been conducted at the outside of a furnace using sodium carbonate or calcium oxide. However, since waste slags produced by this treatment includes large quantities of soda, it needs severe managements to discard. On the other hand, phosphorus has been removed by charging calcium oxide into a converter or by double slag method. However, waste slags produced in the basic oxygen process (hereinafter referred to as converter slag) includes phosphorus so that it cannot be fed into a blast furnace and, further, it needs severe managements to discard.
Also, Japanese Pat. No. 161054 discloses recovering sodium bicarbonate from waste slags produced in the desulfurisation of molten pig iron with sodium carbonate. This method includes the steps of blowing O.sub.2 gas into waste slags to oxidize, effecting extraction with hot water while adding CO.sub.2 gas and oxygen, concentrating the extraction solution and effecting fractional crystallization. However, according to this technique, the waste slag becomes sticky as it breaks down in the oxidation step so that much power is required for stirring and, further, there is required a considerable electric energy to feed an amount of oxygen sufficient for intimately contacting the waste slag with oxygen and, moreover, a long time for reaction is necessary.
Further, processes for recovering alkali from waste slags are disclosed in Japanese Pat. Kokai Nos. 52-148498 and 53-8307, in which the alkali is recovered by treating waste slags with hot water to extract the alkali and feeding CO.sub.2 gas into the extraction solution to deposit the alkali.
However, according to these processes, an extraction rate of alkali is low and a large quantity of hot water and CO.sub.2 gas is necessary and, further, there is disadvantages that the alkali recovered cannot be reused because of being in the form of bicarbonate so that it must be calcined for recycle. Furthermore, with the hot water treatment, the silica and sulphur of waste slag are also extracted and therefore, a step for separating and removing them is necessary. Under such circumstances, it was difficult to recover alkali metals in the form of alkali carbonates from waste slags in an industrial scale.
On the other hand, a process for effecting the desulphurisation of molten pig iron simultaneously with dephosphorisation is disclosed in, for example, Japanese Patent Kokai No. 52-127420. This process requires an amount of sodium carbonate of 4-5 times as compared with the case of effecting the desulphurisation only and therefore, there is an economic problem for practice in an industrial scale. Accordingly, developments of new techniques recovering sodium carbonate are requested for reducing an amount of sodium carbonate to be used and making an economical practice possible.